An image sensor is a device that can convert an optical image into an electronic signal. Image sensors are oftentimes utilized in still cameras, video cameras, video systems, and other imaging devices. Cameras and other imaging devices commonly employ either a charge-coupled device (CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor.
CMOS image sensors include an array of pixels, each of which can comprise a photodetector. CMOS image sensors also include circuitry to convert light energy to an analog voltage. Moreover, CMOS image sensors can include additional circuitry to convert the analog voltage to digital data. Thus, a CMOS image sensor can be an integrated circuit that comprises various analog, digital, mixed-signal, etc. components associated with capturing light and processing imaging related information; accordingly, a CMOS image sensor can be a system on chip (SoC). For example, components integrated into the CMOS image sensor oftentimes include a processor module (e.g., microprocessor, microcontroller, or digital signal processor (DSP) core), memory, analog interfaces (e.g., analog to digital converters, digital to analog converters), and so forth.
Visible imaging systems implemented using CMOS image sensors can reduce costs, power consumption, and noise while improving resolution. For instance, cameras can use CMOS image sensors that efficiently marry low-noise image detection and signal processing with multiple supporting blocks that can provide timing control, clock drivers, reference voltages, analog to digital conversion, digital to analog conversion, key signal processing elements, and the like. High-performance video cameras can thereby be assembled using a single CMOS integrated circuit supported by few components including a lens and a battery, for instance. Accordingly, by leveraging CMOS image sensors, camera size can be decreased and battery life can be increased. Also, dual-use cameras have emerged that can employ CMOS image sensors to alternately produce high-resolution still images or high definition (HD) video.
In a pixel of an image sensor, charge collected in a photodetector is commonly transferred to a floating diffusion region of the pixel. The floating diffusion region can convert the charge to a voltage. Capacitance of the floating diffusion region can determine a voltage gain. For example, a low capacitance of the floating diffusion region can provide a high conversion gain, which can be used for low light scenes. Yet, high conversion gain can be associated with a low dynamic range, particularly in designs with larger pixels. According to another example, a larger capacitance of the floating diffusion region can provide a lower conversion gain, while allowing for a greater dynamic range. However, the capacitance of the floating diffusion region is typically selected during design of the image sensor; accordingly, the conversion gain and the dynamic range of a pixel is typically fixed due to the capacitance of the floating diffusion region being fixed based upon the design of the image sensor.